The present invention relates generally to continuously variable transmission units (“variators”) of rolling-traction type and more specifically to an arrangement for supply of fluid to a roller of such a variator.
Major components of a known toroidal-race rolling-traction variator 10 are illustrated in FIG. 1. Here, two input discs 12, 14 are mounted upon a drive shaft 16 for rotation therewith and have respective part toroidal surfaces 18, 20 facing toward corresponding part toroidal surfaces 22, 24 formed upon a central output disc 26. The output disc is journalled such as to be rotatable independently of the shaft 16. Drive from an engine or other prime mover, input via the shaft 16 and input discs 12, 14, is transferred to the output disc 26 via a set of rollers disposed in the toroidal cavities. A single representative roller 28 is illustrated but typically three such rollers are provided in both cavities. An end load applied across the input discs 12, 14 by a hydraulic end loading device 15 provides contact forces between rollers and discs to enable the transfer of drive. Drive is taken from the output disc to further parts of the transmission, typically an epicyclic mixer, as is well known in the art and described e.g. in European patent application 85308344.2 (published as EP 0185463). Each roller is mounted in a respective carriage 30 which is itself coupled to a hydraulic actuator 32 whereby a controlled translational force can be applied to the roller/carriage combination along a direction generally tangential to the main axis defined by the shaft 16. The actuator 32 comprises a hydraulic piston 34 capable of rotation within its cylinder 36. Such rotation of the piston is associated with a corresponding rotation (or “precession”) of the roller axis about the so-called castor axis, which in the illustrated arrangement is the axis of the piston 34. As the skilled person is well aware, this precession of the roller axis changes the relative diameters of the paths traced out by the roller 28 on the variator discs 12, 14, thereby changing the variator transmission ratio. Because the rollers always seek an orientation in which their axes intersect the axis of the drive shaft 16 they automatically move and precess to positions in which the so-called reaction torque is determined by the biasing force from the actuators 32. The FIG. 1 variator is therefore referred to as being of “torque control” type.
The present invention is however potentially applicable to rolling-traction variators of other types including those known in the field as “part” or “half” toroidal.
In known rolling-traction variators the variator discs do not make direct contact with the rollers. Instead a film of fluid, referred to as “traction fluid”, is maintained between the surfaces of these components and drive is transmitted between them by virtue of shear of this fluid film. Maintenance of the film is of primary importance since direct roller/disc contact would cause excessive wear.
The fluid also has an important function in cooling the variator components, particularly the rollers. In prototype variators the roller operating temperature has been an important factor in determining the power capacity of the variator. The shearing forces in the regions of engagement between variator discs and rollers generate heat, causing the temperature of the rollers (and to a lesser extent of the discs) to rise. Excessive temperatures can (1) damage the rollers themselves (the steel from which they are formed undergoing changes under sustained high temperatures); and (2) impair the performance of the traction fluid, high temperatures and consequent low fluid viscosity leading to a thinner fluid layer between disc and roller, and to higher slip between these components. Sustained high temperatures can also cause the fluid properties to change over time in an undesirable manner.
In practice it is found that the variator's power capacity is limited by the rate of dissipation of heat from the rollers, making improvements in this respect highly important.
An arrangement for supplying traction fluid to the variator rollers is disclosed in our European patent 890044 and its US counterpart U.S. Pat. No. 5,971,885. Here a flow of traction fluid is passed through the actuator/carriage assembly to reach a series of nozzles disposed adjacent to the outer periphery of the roller. A spray of fluid is thus supplied onto the roller periphery.
There are important incentives to increase the efficiency of utilization of the traction fluid. Provision of the required fluid flow requires energy and so reduces overall transmission efficiency; improvements in fluid utilization allow reduction in the flow volume and hence in the corresponding energy requirement. Additionally, studies have shown that the residence time of fluid on the roller surface is typically shorter than is desirable with a view to maximizing conduction of heat from the rollers. An increase in this residence time again offers potential for a reduction in flow volume but also, or alternatively, increases the roller cooling effect and so potentially allows an increase in the power handling capability of the variator and/or a reduction in roller temperature which may increase roller life.
The provision of a shroud in proximity to the roller was proposed in 1938 by W. T. Murdei (U.S. Pat. No. 2,132,751) but to the best of the applicant's knowledge the idea was not taken up in this field and it is believed that because of the form of the shroud—it comprises only a part-circular rim adjacent the roller's outer edge—it would have had limited effectiveness.